1. Field of the Invention
The present invention relates to a lens forming apparatus that presses a material made of glass or resin to form a lens.
2. Description of the Related Art
As methods for manufacturing lenses, a lens manufacturing method using press working and additionally a lens manufacturing method using polishing are known. In the lens manufacturing method using the press working, a material made of glass or resin softened by heating the material made of glass or resin received within a forming die together with the forming die is pressed to form a lens. As a forming die to be used for the press working, there is known a forming die constituted by a trunk die having a through-hole formed therein, a lower die that is fitted into the through-hole from a lower end thereof and allows a material made of glass or resin to be placed thereon, and an upper die that is fitted into the through-hole from an upper end thereof and sandwiches and presses the material made of glass or resin between the upper die and the lower die.
Since the lower die and the upper die move inside the through-hole of the trunk die while at least portions of respective outer walls thereof come into contact with an inner wall of the trunk die, the outer walls of the lower die and the upper die and the inner wall of the trunk die rub each other whereby the forming die is damaged and the lifespan of the forming die becomes short. For this reason, it is preferable to make gaps between the outer walls of the lower die and the upper die and the inner wall of the trunk die wider so that the rubbing between the respective outer walls of the upper die and the lower die and the inner wall of the trunk die does not occur. However, if the gaps are wide, the lower die and the upper die deviate in a lateral direction with respect to a central axis of the through-hole in the vertical direction and move inside the through-hole and the material made of glass or resin is pressed. Thus, an eccentric lens is formed. Additionally, the pressed material made of glass or resin elongates in the direction of the trunk die and flows into the gaps, and thus, burrs are generated.
For example, a lens forming apparatus in which a lower die and an upper die move inside a through-hole of a trunk die while at least portions of respective outer walls thereof come into contact with an inner wall of the trunk die is described in JP2008-273194A. During press forming, a lens is press-formed in a state where all temperatures of the upper die, the lower die, and the trunk die are set to temperatures equal to or higher than a glass transition point.
Additionally, a lens forming apparatus in which a lower die and an upper die move inside a through-hole of a trunk die while at least portions of respective outer walls thereof come into contact with an inner wall of the trunk die is also described in JP2004-351740A. However, during the press forming, a lens is press-formed in a state where one or both of the upper die and the lower die are heated and thereby thermally expand, the trunk die is not heated but is kept from thermally expanding, and the coefficients of linear expansion of the upper die and the lower die are greater than the coefficient of linear expansion of the trunk die. For this reason, during the press forming, gaps between outer walls of the lower die and the upper die and an inner wall of the trunk die can be narrowed.